Typically, piston-cylinder devices employ metal rings with oil lubrication to provide a seal. In the absence of lubricating oil, a piston seal can be constructed from a self-lubricating material such as a polymer or ceramic (e.g., graphite). Use of a self-lubricating material for the seal eliminates scuffing or galling caused by an unlubricated metal ring but can result in a relatively high wear rate as compared to a conventional oil-lubricated seal arrangement.
For economic reasons, it is desirable for the seal to function for as long as possible before needing replacement. For example, a typical target might be hundreds or thousands of hours of operation. Throughout this lifetime the seal wears down radially. To compensate for this wear, the seal is typically split in one or more places, allowing pressure to expand the ring outward and maintain sealing contact with the cylinder wall in spite of the material removed via wear.
As the seal wears, it stretches to conform to the cylinder during, for example, a piston stroke. This stretch results in increasing levels of tensile stress in the seal, with the highest stretch-induced tensile stresses typically arising at the inner diameter (ID) of the ring or ring segment. For materials that are weak in tension (i.e., brittle) this can be a life-limiting phenomenon wherein the seal is capable of only a finite amount of stretch (and in turn a finite amount of radial wear) before tensile stresses cause it to fracture.